We intend to study the induction of specific immunological unresponsiveness in cultures of lymphoid cells from rabbits and the transfer of such unresponsiveness to other cultures by suppressor cells. The model is a primary in vitro antibody response to solubilized phage antigens which is free of demonstrable requirements for help by either macrophages or T-lymphocytes and which is entirely IgM. We shall examine two putative mechanisms leading to unresponsiveness, namely B-cell receptor blockade and the induction of T-suppressor activity. Hypotheses to be tested include one that visualizes immunity and tolerance induction as events that occur simultaneously in response to a wide dose of antigen concentrations and another which considers B cells with blocked receptors a particularly effective inducer of specific T-suppressor cells. The in vitro system lends itself particularly well to the cell separation and reconstitution experiments that need to be done for a better insight into immunoregulatory processes which, in turn, will be needed for improvements in the understanding and practice of immunological manipulations in allergy, autoimmune disease and cancer.